Distinguishing Between Salt-related and Algal Structures: A 3D Seismic Example from the Northwest Denver Basin

Antara Goswami, Katie Joe McDonough, and Brian W. Horn
ION Geophysical

Paleozoic deposits in the Denver Basin include evaporites as well as shallow-shelf carbonate and algal sediments, which form in restricted marine settings. Anomalous domal features observed in 3D seismic may represent (a) Permo-Pennsylvanian carbonate mound buildups, (b) evaporate dissolution/flowage, or (c) tectonic structures generated by differential movement along basement faults. Deep-seated domal algal mounds have been an exploration target in many US basins and are difficult to differentiate from salt-related features or basement structures in a seismic section, especially without well control. Some of the alleged 'algal' structures drilled in the northwest Denver Basin were later identified as salt. This paper is a practical exercise in experimentally selecting the best distinguishing parameters to reduce reservoir risk in carbonate exploration. For example, differential movement on basement faults may result in convergent/divergent strata which mimic mound 'growth'. Similarly, reflection coefficient alone is not a reliable parameter to evaluate a domal feature of unknown internal composition. Low density salt structures generate a seismic trough at the top in normal polarity data. However, porous carbonate mounds may also exhibit the same signature. These uncertainties are addressed using tactical analysis methods. We analyzed stratigraphic data, such as the nature and direction of onlapping sediments around the structure, and the internal and external architecture of the 'mound feature'. Geophysical attribute analyses aided us in reconstructing the stratigraphic evolution of the feature. Co-rendering attributes allowed us to map internal horizons and compare related parameters. A velocity model assisted in eliminating the possibility of salt-generated velocity anomalies at the level of subsalt reflectors. This approach is designed to reduce uncertainty in predicting reservoir facies and develop a potential exploration play type in Rocky Mountain basins.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013